Phenalenyl-based open-shell polycyclic aromatic hydrocarbons.

Personal Account DOI: 10.1002/tcr.201402065

THE CHEMICAL RECORD

Phenalenyl-Based Open-Shell Polycyclic Aromatic Hydrocarbons Takashi Kubo Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan) E-mail: [email protected]

Received: July 14, 2014 Published online: October 24, 2014

ABSTRACT: The phenalenyl radical is a polycyclic aromatic hydrocarbon (PAH) radical. Owing to its widely distributed spin structure, phenalenyl is relatively stable compared to other hydrocarbon radicals and has been studied from the viewpoint of its application to electroconductive and magnetic materials. In addition, a strong intermolecular spin–spin coupling nature is another feature of phenalenyl. This account summarizes my studies so far into PAH radicals containing the phenalenyl scaffold in terms of their amphoteric redox properties and singlet biradical character, which strongly rely on the characteristic electronic structure, that is, non-bonding character and sixfold symmetry of a singly occupied molecular orbital of the phenalenyl radical. Keywords: aromaticity, hydrocarbons, radicals, phenalenyl, polycycles

1. Introduction It is my great pleasure that I could contribute to the publication of Professor Tetsuo Nozoe’s Autograph Books. Honestly speaking, I met Professor Nozoe only two or three times, but I clearly remember him eagerly discussing azulene chemistry with Professor Ichiro Murata in Murata’s laboratory when I was a master’s student there. I am honored that my academic research is thoroughly based on their novel aromatic chemistry. My research interests are to explore fascinating new functions of open-shell compounds and also to investigate a new aspect of chemical bonds, based on the syntheses and physicochemical measurements of novel organic radicals. I am interested in many kinds of π-conjugated radicals and the phenalenyl radical plays a central role. In this account, I firstly give an overview of the general principles of phenalenyl and then introduce three chemists who have greatly contributed to the chemistry of phenalenyl. In the main part, I would like to summarize my studies on phenalenyl so far.

Chem. Rec. 2015, 15, 218–232

1.1. Features of Phenalenyl Phenalenyl is a thermodynamically stabilized neutral hydrocarbon radical that has long attracted the attention of experimental and theoretical organic chemists (Figure 1).[1,2] Due to its unique electronic structure, many efforts have been devoted to studies on its chemical reactivity, spectroscopic analyses, electroconductive behavior, and magnetic properties. According to the theorem of Coulson–Rushbrooke–LonguetHiggins,[3–5] phenalenyl should possess one non-bonding molecular orbital (NBMO), which distributes only on the starred carbon atoms, because the difference in the numbers of starred and unstarred carbons is one. An exception is the central carbon atom, which bears no coefficient, because putting an unpaired electron or charges here results in antiaromatic 12π cyclic conjugation in the periphery of the molecule. In this way, the characteristic MO distribution of phenalenyl can be easily predicted by a completely

© 2014 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Wiley Online Library 218

Personal Account

THE CHEMICAL RECORD

Fig. 1. Phenalenyl radical.

trivial “back of the envelope” procedure and even a high-level quantum chemical calculation gives the identical result. The NBMO of phenalenyl is occupied by one electron in the neutral state (Figure 2). Due to the non-bonding character of the singly occupied MO (SOMO), all the redox states (cation, neutral radical, and anion) of phenalenyl possess the same π-electron delocalization energy under the Hückel MO (HMO) approximation, and the phenalenyl radical is therefore expected to behave as a good amphoteric redox system. This high redox ability is one of the major characteristics of phenalenyl, and phenalenyl-based radicals have been extensively studied as building blocks for molecular electroconductors.[6] Another feature of phenalenyl is large spin polarization in the neutral radical state owing to the non-bonding character of the SOMO. Spin polarization is a key mechanism for ferromagnetic spin alignment, which is known as McConnell’s first model of magnetic interaction through space.[7] Theoretical study by Yamaguchi revealed the possibility of ferromagnetic coupling in a slipped stack of phenalenyl radicals.[8] A more recent topic in phenalenyl research is to understand chemical bonding interactions

Takashi Kubo was born in Yamaguchi, Japan, in 1968. He graduated from Osaka University in 1991, received his M.Sc. in 1993 under the guidance of Professor Ichiro Murata, and received his Ph.D. in 1996 under the guidance of Professor Kazuhiro Nakasuji. After working at Mitsubishi Chemical Co., he joined Professor Nakasuji’s group at the Department of Chemistry, Graduate School of Science, Osaka University, in 2000 as Assistant Professor. In 2006 he served as Associate Professor. Since 2006 he has been a Professor of the Graduate School of Science, Osaka University. He received the Bulletin of the Chemical Society of Japan Award in 2001. His research interests are structural and physical organic chemistry, mainly the syntheses and properties of polycyclic aromatic compounds with open-shell character, and the development of multifunctional materials using a cooperative proton and electron transfer system.

Chem. Rec. 2015, 15, 218–232

Fig. 2. Molecular orbitals of the phenalenyl radical calculated by a HMO method.

within a π dimer of phenalenyl radicals. Kochi[9] and Kertesz[10] have greatly contributed to this research and their theoretical works are associated with unraveling the nature of intermolecular interactions of organic radicals. 1.2. Murata’s Phenalenyl Ichiro Murata (emeritus professor of Osaka University, Japan) is a pioneer of phenalenyl chemistry in Japan. Murata read Professor Nozoe’s review entitled “The Study on Hinokitiol”[11] when he was a high school student and became interested in its unique seven-membered structure and aromatic behavior with great surprise. As a student in the research group of Professor Nozoe, in which the new field of non-benzenoid aromatic chemistry was being developed, Murata started his research on the chemistry of seven-membered π-conjugated systems and received his Ph.D. degree in Science from Tohoku University in 1960 for studies on the synthesis and chemical reactivity of 1,3-diazaazulene, which were published in J. Am. Chem. Soc. in 1954.[12] He continued to study π-conjugated systems as an assistant professor in the research group of Professor Kitahara, especially focusing on fulvalenes including calicene, and in 1967, Murata was appointed to a professorship at Osaka University. At Osaka University, Murata focused his efforts on research directed toward understanding electron delocalization and aromaticity by using a wide variety of π-conjugated compounds: triptycenes, azulenes, heteropines, and phenalenyls. Among them, phenalenyl was his primary interest and he investigated the effect of a phenalenyl scaffold on π-conjugated systems through chemical reactivity, NMR,


www.tcr.wiley-vch.de 219

Personal Account

THE CHEMICAL RECORD

X X = O or S PdL2

1

2 X

+

–

L2Pd PdL2

+

Fig. 4. Amphoteric redox systems based on bisphenalenyl.

–

Fig. 3. A part of Murata’s works on phenalenyl.

UV–vis measurements, and so on. For instance, compound 1 is polarized so as to bear positive and negative charges on the phenalenyl and cyclopentadienyl moieties, respectively,[13] whereas the charges in 2 are inverted (Figure 3).[14] Also noteworthy is an experimental observation of threefold-degenerate inter-ring migration of a Pd atom in a Pd(η3-phenalenyl) complex.[15] Thus, Murata was committed to investigating the chemical characteristics of phenalenyl.

Fig. 5. Kinetically stabilized phenalenyls.

1.3. Nakasuji’s Phenalenyl Kazuhiro Nakasuji (emeritus professor of Osaka University) received his Bachelor degree from Osaka University in 1965. Under the guidance of Professor Masazumi Nakagawa, he pursued graduate study and received his Ph.D. degree for studies on the synthesis and characterization of polyynes in 1970.[16,17] He joined Professor Murata’s research group as an assistant professor in 1971 and focused on the aromaticity of novel π-conjugated systems. In 1977–1979 he worked in the research group of Professor Ronald Breslow and this was a particularly exciting period of time for Nakasuji. He experienced cutting-edge physical organic chemistry in the Breslow group, which shifted his interests to electron delocalization in molecular assemblies, that is, intermolecular π conjugation. After returning to Osaka University, he started a new project toward the development of novel electroconductive materials that are not based on tetrathiafulvalene (TTF). The most extensively studied were amphoteric redox systems based on bisphenalenyl (Figure 4), which were expected to behave as good building blocks for electroconductors because the bisphenalenyls are easily oxidized and reduced to afford stable cation and anion species.[18] Furthermore, Nakasuji succeeded in the first isolation of the phenalenyl radical in crystalline state by introducing three tert-butyl groups in 1999 (Figure 5).[19] This radical forms a π

Chem. Rec. 2015, 15, 218–232

Fig. 6. Phenalenyls aiming for single-component electroconductors.

dimer in the crystal and opened the door to the study of multicenter bonding[20] (pancake bonding),[21] which is a new mode of chemical bonding of organic radicals. He also investigated the effect of heteroatoms on phenalenyl by using heteroatom-containing analogues, in collaboration with Professor Morita.[2] 1.4. Haddon’s Phenalenyl Robert C. Haddon (professor of University of California, Riverside) has made a great contribution to understanding the electroconductive properties of phenalenyl. In 1975, he pointed out the possibility that phenalenyl is a good candidate for single-component molecular electroconductors.[22] Based on his own idea, Haddon has pursued the realization of single-component organic metals by preparing and investigating phenalenyl-based radicals including perchloro[23] and hexathio[24] derivatives (Figure 6). The most successful compounds are spiro-conjugated bisphenalenyls and indeed a cyclohexane derivative shows metallic behavior.[25]



Personal Account

THE CHEMICAL RECORD

3 Fig. 7. Tribenzodecacyclenyl (3). Fig. 9. Seven redox states of 3.

HMO approximation. If Hund’s rule is applicable,[26] a monoanion species of 3 (3−) is expected to be triplet biradical in the ground state. At that time, studies on molecular magnets were a cutting-edge field in chemistry and physics, and according to McConnell’s second model,[27] triplet species were expected to be promising components for ferromagnetic materials. Other partially occupied redox states also include the important issue of whether spins and charges are delocalized or localized in relation to the Jahn–Teller distortion.[28] Considering the application for molecular magnets, I started my research with the synthesis of the parent 3. The starting material was decacyclene, which could be cheaply purchased from Aldrich (now commercially unavailable). My synthetic plan involved carbon-increasing reactions of decacyclene followed by ring cyclization to construct the phenalenyl skeleton. However, I was distressed by the very low solubility of the synthetic intermediates and, furthermore, a trihydro precursor of 3 was very reactive to oxygen to give rise to partially oxidized radicals. Due to severe broadening of the 1H NMR signals, I could not determine the structure of the oxidized products. Fig. 8. Selected molecular orbitals of 3 calculated by the HMO method.

2.2. Hexa-tert-butyl Derivative

2. Tribenzodecacyclenyl: Starting Point of My Research Works 2.1. Theoretical Background I started my research life as a Bachelor student in Professor Murata’s group in 1990. My research theme given by Murata was to elucidate the spin multiplicity of tribenzodecacyclenyl (3, Figure 7) in its monoanion state. The interesting point regarding 3 lies in the variety of electronic and spin structures in seven redox states. A simple HMO calculation for 3 presents quasi triply degenerated frontier orbitals (ψ22–ψ24 in Figure 8), which are derived from how three phenalenyl radicals weakly couple through a benzene ring. When these three MOs are completely unoccupied, 3 becomes a trication state, and when they are fully occupied, a trianion state (Figure 9). The most interesting state is the monoanion, in which two degenerate MOs (ψ23 and ψ24) are occupied by two electrons under the

Chem. Rec. 2015, 15, 218–232

After Professor Murata retired from Osaka University in 1992, I continued my research under the guidance of Professor Nakasuji. With his advice, I focused my efforts primarily on research directed towards understanding the electronic structure of 3 in seven redox states (Figure 9). I decided to introduce tert-butyl groups on 3 to increase solubility and stability (Figure 10). In the first step of the synthetic route to the target compound, tert-butyl groups were introduced to decacyclene by Friedel–Crafts alkylation using tert-butyl chloride with AlCl3 to give rise to a hexa-tert-butyl derivative. The attachment of tert-butyl groups led to a drastic improvement in solubility, which allowed subsequent reactions to be carried out under “normal” conditions. A trihydro precursor of the target compound was found to be stable in air. Seeing the very simple 1H NMR spectrum of the precursor, I felt that my hard work for four years had definitely paid off.



Personal Account

THE CHEMICAL RECORD

+ / 2+ 2+ / 3+ –/•

•/+

2– / – 3– / 2–

Fig. 10. (Left) Hexa-tert-butyl derivative of tribenzodecacyclenyl (4). (Right) ESR spectrum of 4.

E3ox E2ox

E1ox

E1red E2red E3red

+0.87 +0.68 +0.27 –0.51 –0.89 V vs SCE

–1.25

Fig. 12. Cyclic voltammogram of 4.

Fig. 11. Dynamic Jahn–Teller distortion of 3.

Because the precursor could be isolated in a pure form, I tried to generate the target neutral radical (4) under oxygenfree conditions by dehydrogenation with p-chloranil. Surprisingly, 4 was found to be so stable as to allow its purification by silica gel column chromatography in air. The neutral radical state possesses only one electron in doubly degenerate orbitals (ψ23 and ψ24). In this case, Jahn–Teller theory suggests that the molecular skeleton could distort from C3 symmetry to C2, and then an unpaired electron localizes on one phenalenyl moiety. However, the ESR measurement of a toluene solution of 4 showed that the unpaired electron delocalizes equally on the three phenalenyl moieties. Presumably, the C3 structure arises from the dynamic Jahn–Teller effect, that is, a rapid equilibrium between C2 distorted structures (Figure 11), being supported by a broad ESR signal.[29] As mentioned above, the interesting point about 3 lies in the variety of electronic and spin structures in seven redox states. Accessibility to all the redox states can be estimated by cyclic voltammetry. The cyclic voltammogram of 4 gave six redox waves with high reversibility and the difference in potential between the third oxidation wave (E3ox) and the third reduction wave (E3red) was only 2.12 V (Figure 12). These results suggest that all the redox states can be readily generated by control of the redox reaction conditions.[30] The terminal redox states, that is, the trication (43+) and trianion (43−) species, were generated with no difficulty by oxidation with D2SO4 and reduction with a potassium mirror, respectively. Both of the supercharged species were persistent in a degassed tube even at room temperature. 1H NMR spectra gave very simple patterns consistent with a C3 symmetric struc-

Chem. Rec. 2015, 15, 218–232

ture. The monoradical dication (4•2+) should have a nondegenerate SOMO (ψ22) with a C3 symmetric MO distribution pattern. The ESR spectrum of 4•2+, which was generated by oxidation with SbCl5, supports the highly delocalized structure of an unpaired electron. On the other hand, the pair of the relevant frontier orbitals (ψ23 and ψ24) of monoradical dianion (4•2−) is degenerate or nearly degenerate, with an occupancy number of three. The ESR spectrum of 4•2−, which was generated by short contact of 4 with a potassium mirror, revealed that the unpaired electron mainly localizes on one phenalenyl moiety. This Jahn–Teller distortion would be caused by ion pairing of countercations (K+) that reside near two phenalenyl moieties. The prime interest in the monoanion (4−) relates to whether this species is singlet or triplet. ESR measurements afforded no signal attributable to a triplet species in the samples at 77 K and 173 K. Although the absence of the triplet ESR signals does not prove the singlet ground state of the monoanion, ion pairing of K+ would be responsible for the spin multiplicity. In this way, the study of tribenzodecacyclenyl was a main part of my Ph.D. thesis.

3. Phenalenyl 3.1. Tri-tert-butyl Derivative Because we noticed that steric protection by tert-butyl groups led to a great improvement of stability beyond our expectations, we decided to isolate the phenalenyl radical by introducing tert-butyl groups. At that time, the phenalenyl radical had only been characterized by spectroscopic methods such as ESR,[31] ENDOR, and TRIPLE,[32] or by electrochemical analysis.[33]



Personal Account

THE CHEMICAL RECORD

Fig. 15. Generation of 7 and its σ dimer. Fig. 13. Tri-tert-butyl phenalenyl (5) and Hafner’s cyclohepta[def]fluorene (6).

reflection spectra of a single crystal of 5 with the help of Yakushi (Institute for Molecular Science, IMS). The transition moment of the band was polarized along the direction connecting the center of the phenalenyl rings. This finding suggests that the origin of the band is an electronic transition between HOMO and LUMO, which are newly formed by the orbital interaction of a pair of SOMO in the π dimer. 3.2. Tris(pentafluorophenyl) Derivative Fig. 14. ORTEP drawing of 5. Hydrogen atoms are omitted for clarity. (Left) Top view. (Right) Side view.

The SOMO of phenalenyl distributes only on the α positions and we expected that tert-butyl groups introduced at β positions could effectively protect the reactive α carbons (Figure 13). Although two tert-butyl groups could be introduced easily by general synthetic methods, we met with difficulty in the introduction of the third. Three months were required for successful introduction of the tert-butyl group, a clue for which came from Hafner’s literature describing the synthesis of a tri-tert-butyl derivative of cyclohepta[def]fluorene (6).[34] The key reaction was a Reformatsky reaction with methyl 2-bromo-3,3-dimethylbutanoate. The generation of 5 was performed in a sealed degassed tube by the reaction of a hydro precursor with p-chloranil. Contact of a hexane solution of the precursor with solid p-chloranil at 60°C caused the gradual color change of the solution from colorless to blue, and eventually a deep-blue solution was obtained. Allowing this blue solution to stand at 4°C for one week afforded dark blue prisms and X-ray crystallographic analysis revealed that the crystals were indeed 5.[19] Surprisingly, 5 formed a face-to-face π dimer, in which two phenalenyl radical π planes were superimposed at a separation distance of 3.2–3.3 Å (Figure 14). The solid-state UV measurement showed an intense broad band at 612 nm, which is responsible for the blue color. Initially, however, the intense band could not be assigned by using a ZINDO/S calculation, because the calculation predicted a forbidden band at around 500 nm as the lowest-energy band. I suspected that the band would originate not from a single molecule but from a π dimer, and therefore measured polarized

Chem. Rec. 2015, 15, 218–232

As Haddon predicted in 1975,[22] a one-dimensional (1D) chain of equidistantly stacked monoradical species forms a half-filled band, which is a requirement for metallic behavior. Attempts to form an ideal 1D chain based on phenalenyl were initially carried out by Haddon using the perchlorophenalenyl radical. However, bulky chlorine atoms induced deformation from the planar structure and nonuniform stacking was also observed. We also tried to create an ideal 1D chain by utilizing the strong electrostatic interaction of pentafluorophenyl groups that were introduced at β positions. A hydro precursor was prepared in ten steps from a commercially available 2,8-dibromonaphthalene and dehydrogenation with DDQ resulted in the formation of not the target radical (7) but a σ dimer of it (Figure 15). We were disappointed with this result, but following Morita’s work on 1,3-diazaphenalenyl, which transformed from a σ dimer to a π dimer upon heating in the solid state,[35] we also heated a solid of the σ dimer at 300°C in a degassed tube. The heating resulted in melting to a liquid accompanied by a color change from yellow to purple and, surprisingly, cooling the liquid afforded relatively large needles suitable for X-ray crystallographic analysis. X-ray measurement at 10 K showed the distinctive feature that 7 forms a 1D chain with equidistant stacking of the molecule, as shown in Figure 16.[36] The electrical conductivity of a compressed pellet of the purple 7 was found to be less than 10−10 S cm−1 at room temperature. Due to a large U/W value, where U and W are the on-site Coulomb repulsion energy and bandwidth, respectively, 7 becomes a Mott–Hubbard insulator. Although the 1D chain possesses a relatively large anti-ferromagnetic interaction of 2J/kb = 600 K, no spin-Peierls transition was observed even at 10 K. The persistency of 7 to the spin-Peierls transition relies on two structural factors of the C6F5 substituent groups: a



Personal Account

THE CHEMICAL RECORD

Fig. 16. ORTEP drawing of (left) a σ dimer of 7 and (right) a 1D stack of 7.

Fig. 18. ORTEP drawing of the butyl derivative of 8.

4. Singlet Biradicals Based on Bisphenalenyl 4.1. Theoretical Background of Singlet Biradicals

Fig. 17. (a) Resonance formula of 8. (b) SOMO of 8 calculated by a HMO method. (c) Butyl derivative of 8.

slipped stacking arrangement and a twisted conformation with respect to the phenalenyl ring. 3.3. π-Extended Phenalenyl Generally, hydrocarbon radicals are very reactive compared to heteroatom-centered radicals and the introduction of bulky substituents, that is, kinetic stabilization, is necessary for their isolation in a pure form. On the other hand, thermodynamic stabilization by spin delocalization should also be effective to suppress the reactivity of hydrocarbon radicals so as to allow them to survive in a discrete form in the solid state even without bulky substituents. I designed a new hydrocarbon radical (8) in which an unpaired electron delocalizes over two phenalenyl rings (Figure 17). Actually, this research project was conducted when I stayed for three months (June to August, 2007) in Haddon’s group as a visiting researcher. Fortunately, I succeeded in the isolation of a cation species of 8, and after coming back to Japan, I continued this research and finally isolated an n-butyl derivative of 8 in a crystalline form. X-ray crystallographic analysis revealed that the radical exists in a discrete form in the solid state (Figure 18).[37] This is the first hydrocarbon radical that can survive in the solid state only by thermodynamic stabilization. The half-life in air determined at room temperature was almost 60 h, which is in contrast to the rapid reaction of the phenalenyl radical with oxygen, suggesting that spin delocalization is very effective for the stabilization of organic radicals.

Chem. Rec. 2015, 15, 218–232

When two phenalenyl radicals are coupled through a conjugated system, two NBMOs split into a bonding and an antibonding MO. The extent of splitting of the two NBMOs (that is, HOMO–LUMO splitting) is closely related to the singlet biradical character of the molecule. In the case of a very large gap, the molecule possesses closed-shell character and any magnetic behaviors cannot be observed. When the gap becomes smaller, the nature of the molecule changes progressively from closed-shell singlet to open-shell singlet. The simplest model for this change is a homolytic dissociation of the hydrogen molecule H2. Upon stretching, the spatial overlap of two NBMOs becomes small, leading to a smaller HOMO–LUMO gap, and the mixing of HOMO and LUMO gives open-shell character to the stretched H2. Biradical character can be estimated by quantum chemical calculations. The extent of the mixing of HOMO and LUMO is used as a measure of the biradical character. Restricted Hartree–Fock (RHF) and two-configuration self-consistent field (SCF) calculations are simple but reliable methods for the estimation. By using this approach, the Coulomb interaction between two unpaired electrons is taken into consideration and the two unpaired electrons are permitted to occupy a different part of space. The occupation number of the LUMO, which can be estimated by a complete active space SCF (CASSCF) calculation, is used as an index of biradical character. Another effective approach is the broken-symmetry (BS) formalism based on the unrestricted Hartree–Fock (UHF) wave function, which was introduced by Noodleman.[38] The occupation number of the lowest unoccupied natural orbital (LUNO), which is referred to as NOON,[39] is a good measure of biradical character. However, the BS method is intrinsically accompanied by the spin contamination problem, and



Personal Account

THE CHEMICAL RECORD

Fig. 19. (a) Resonance formula of 9. (b) Diisopropyl derivative of 9.

therefore a biradical index that removes the spin contamination from the BS solution by applying a spin-projection method was defined by Yamaguchi.[40–42] The following bisphenalenyl compounds have been studied in terms of singlet biradical character. Central π-conjugated systems (quinoid systems) function as a generator of singlet biradical character and the two terminal phenalenyl rings act as a spin-delocalizing unit (that is, a thermodynamic stabilizer). 4.2. Tetra-tert-butyl Derivative of IDPL: Investigating Singlet Biradical Character in a Single-Molecule State IDPL (9), which consists of p-quinodimethane and two phenalenyl rings, was first designed by Nakasuji as an amphoteric redox compound (Figure 19). Indeed, a diisopropyl derivative of 9 shows a very small energy gap (ΔE1redox = 1.10 V) between the first oxidation (E1ox) and first reduction (E1red) potentials.[43] From the small electrochemical HOMO–LUMO gap, 9 is expected to behave as a singlet biradical species. The quinoid Kekulé form of 9 would resonate well with the biradical form 9′ as a result of gaining the aromatization energy of the central six-membered ring, and then the unpaired electrons emerging on the terminal carbons of the p-quinodimethane moiety can delocalize on the phenalenyl rings (the form 9″). This resonance consideration leads to the conclusion that 9 should be a thermodynamically stabilized singlet biradical. The biradical character (y) of 9 estimated from the NOON value by a B3LYP/6-31G** method is 37%. For the purpose of thoroughly investigating the singlet biradical character of 9, four tert-butyl groups were introduced on the phenalenyl rings in order to increase solubility and stability and to make the molecule discrete in the solid state. The tetra-tert-butyl derivative (10) was prepared in 16 steps from acenaphthene and isolated in pure form. Compound 10 gave very broad 1H NMR signals in the aromatic region at room temperature, and upon cooling, progressive line

Chem. Rec. 2015, 15, 218–232

Fig. 20. (Left) Tetra-tert-butyl derivative of IDPL (10). (Right) Variabletemperature 1H NMR spectra of 10.

sharpening was observed (Figure 20).[44] This behavior is frequently observed for singlet biradical species. Signal broadening implies the presence of magnetic species, but singlet biradical character itself is temperature independent. Actually, weak coupling of unpaired electrons within the molecule leads to a small energy gap (ΔES–T) between the singlet ground state and the excited triplet state, and thermally excited triplet species cause the signal broadening at elevated temperatures (the relationship between singlet biradical character and ΔES–T is slightly complicated, see Eq. (1) in Section 4.6). The triplet species was indeed detected by a solid-state ESR measurement, which gave signals with a zero-field parameter of D = 9.6 mT and E ≤ 0.2 mT. Least-squares curve fitting of the signal intensities at various temperatures gave the ΔES–T value of 20.4 kJ/ mol, from which we could determine the intramolecular covalent bonding interaction of 20.4 kJ/mol. It is noted that this value is almost 8% of the rotational barrier of the ethylene C=C double bond (270 kJ/mol).[45] 4.3. Thienoquinoid Bisphenalenyl (TDPL): Inspiration for Intermolecular Covalent Bonding Interaction TDPL (11), which is a bisphenalenyl incorporating a thienoquinoid skeleton, possesses very large singlet biradical character (y = 72%) and a tetra-tert-butyl derivative of TDPL (12) was prepared by multistep synthesis (Figure 21).[46] The large biradical character is consistent with the experimental result that 12 gave no sharp 1H NMR signal in the aromatic region even at −90°C (Figure 22). Compound 10 showed a very intense low-energy absorption band (ε = 369,000) at 755 nm, whereas 12 gave a weak and broad band (ε ≈ 3,000) in the NIR region (Figure 23). This difference in appearance of the lowest-energy bands arises from the extent of coupling of two unpaired electrons. Weaker coupling, that is, larger singlet biradical character, leads to



Personal Account

R

R

THE CHEMICAL RECORD

R

S

R

R

R

R

S

R

R = H, 11 R = tBu, 12

Fig. 21. Thienoquinoid bisphenalenyl, TDPL (11), and its tetra-tert-butyl derivative (12).

Fig. 23. Solution UV–vis–NIR spectra of 10 (dashed line) and 12 (solid line).

Fig. 24. ORTEP drawing of 12. (a) Top view and (b) side view.

Fig. 22. Variable-temperature 1H NMR spectra of 12.

more localization of two unpaired electrons in separated space. The spatial overlap of electron wave functions becomes smaller, and then the HOMO–LUMO splitting becomes smaller, and at the same time, the transition probability of an electron from HOMO to LUMO also declines. The most distinctive feature related to singlet biradical character is a highly bent structure in a dimeric pair of 12 in the solid state, as shown in Figure 24. Substantially short nonbonding contacts of ∼3.1 Å were observed between the thiophene rings, whereas the terminal rings were separated by over 4.2 Å due to the steric repulsion between tert-butyl groups and six-membered rings. Attractive interaction in the short π–π contact would be derived from an intermolecular covalent bonding interaction of unpaired electrons. In other words, singlet biradical 12 wants a covalent bond between molecules. 4.4. Diphenyl Derivative of IDPL: A New Aspect of Singlet Biradical Species For the purpose of elucidating the electronic structure of singlet biradicals in terms of open-shell character, many efforts

Chem. Rec. 2015, 15, 218–232

have been devoted to spectroscopic investigations and products analysis as well as theoretical calculations.[47–53] In general, singlet biradicals possess common features such as a small gap between first oxidation and first reduction potentials, a longwavelength absorption band, NMR signal broadening, and magnetic susceptibility increasing at elevated temperatures. However, studies of singlet biradicals frequently encounter difficulty in the direct observation of spin structure, because of the intrinsic spin-pairing nature. Then, what are the unique characteristics of singlet biradicals? How can we detect them experimentally? The close contact in the dimer of 12 affords a clue to these questions. Because of the weak intramolecular covalent bonding interaction, 12 demands an intermolecular covalent bonding interaction, eventually forming the dimer. The tert-butyl groups disturb π–π contact between the phenalenyl rings, on which unpaired electrons mostly reside, and therefore 12 had no choice but to adopt a bending form to secure an intermolecular covalent bonding interaction. If no bulky substituent is attached to the phenalenyl rings, we might be able to obtain a molecular aggregate with a very close π–π overlap between phenalenyl rings. Based on that working hypothesis, I designed a diphenyl derivative (13) of



Personal Account

THE CHEMICAL RECORD

Fig. 27. Resonance formula of 13, and 7,14-diphenylacenaphtho[1,2k]fluoranthene (14). Fig. 25. (Left) Diphenyl derivative of IDPL (13). (Right) Variabletemperature 1H NMR spectra of 13.

Fig. 26. (a) One-dimensional stack of 13. (b) Top view of overlapping phenalenyl rings. (c) SOMO–SOMO overlap of the π dimer of phenalenyl radical.

IDPL 9 (Figure 25). Compound 13 was prepared by multistep synthesis and could be isolated in a crystalline form. X-ray analysis revealed that 13 formed a 1D chain in a slipped stacking arrangement with an average π–π distance of 3.14 Å, which is substantially shorter than the sum of the van der Waals radius of a carbon atom (3.4 Å).[54] The π–π overlap was found only on the phenalenyl ring (Figure 26) in the same arrangement as that of the π dimer of 5. In the π dimer of the phenalenyl radical, a staggered stacking maximizes a SOMO– SOMO interaction (Figure 26c), and the very effective overlap of SOMOs is one of the crucial attractive bonding interactions. Therefore, the attractive force in the 1D stack of 13 would originate from the intermolecular covalent bonding interaction of unpaired electrons, in addition to a conventional dispersion force. In the 1D stack of 13, the length of the bonds connecting the phenalenyl rings and the benzene ring (denoted by a in Figure 27) is longer than that of the tetra-tert-butyl derivative (10) that adopts a discrete form in the solid state, but is still shorter than that of the corresponding bonds of 7,14diphenylacenaphtho[1,2-k]fluoranthene (14):[55] 1.470(3) Å (13), 1.450(4) Å (10), 1.479(2) Å (14). The length of the bond a in 13 is sensitive to the extent of the intramolecular covalent

Chem. Rec. 2015, 15, 218–232

Fig. 28. UV–vis–NIR spectra of 13. Absorption spectrum in CH2Cl2 (dashed line), and optical conductivity (solid line) obtained with light polarized along the stacking direction.

bonding interaction of two unpaired electrons, because the bond a has both single- and double-bond character in the quinoid Kekulé form but only single-bond character in the biradical form. On the other hand, only a single bond can be drawn in 14 and, consequently, the length of the bond a in 14 would be equivalent to that in a perfect biradical state of 13. Therefore, the intermediate bond length of 13 implies that the covalent bonding interaction is operative not only between molecules but also within a molecule. The optical spectrum of 13 in the solid state showed a drastic red-shift of the lowest-energy band with respect to the solution band (Figure 28), due to the long-range π conjugation through the intra- and intermolecular covalent bonding interactions of electrons. The shift amounts to ∼6600 cm−1. The large red-shift due to the coexistence of the couplings implies that the 1D chain of 13 mimics the electronic structure of an infinite polyene, although the magnitude of the interaction of unpaired electrons is quite different.



Personal Account

THE CHEMICAL RECORD

Fig. 30. 1D chains of (a) 19 and (b) 20.

Fig. 29. NDPLs (15, 17, 19) and ADPLs (16, 18, 20).

The strength of the covalent bonding interactions was quantum-chemically estimated by Huang and Kertesz.[56] They interpreted the electronic structure of the 1D chain using an alternating Heisenberg chain model and, interestingly, found that the intermolecular interaction (3400 K, = 28 kJ/mol) is stronger than the intramolecular one (2300 K, = 19 kJ/mol). 4.5. Naphthoquinoid and Anthraquinoid Bisphenalenyls (NDPL and ADPL): Stronger Intermolecular Covalent Bonding Interactions Replacement of the central benzene ring in IDPL (9) with naphthalene or anthracene rings will lead to enhanced singlet biradical character (Figure 29), because naphthalene and anthracene have larger aromatic stabilization energies than benzene. A broken-symmetry UB3LYP/6-31G** calculation showed that NDPL (15) and ADPL (16) possess 56% and 62% of singlet biradical character in the ground state, respectively. Intramolecular covalent bonding interactions of the two unpaired electrons in 15 and 16 were assessed with di-tertbutyl tetraphenyl derivatives 17[57] and 18.[58] The SQUID measurements of powdered 17 and 18 revealed smaller singlet– triplet energy gaps (16 kJ/mol for 17, ∼8 kJ/mol for 18) than that of 10. The HOMO–LUMO gaps of 17 and 18 are also smaller than that of 10, with values of 755 nm, 865 nm, and 984 nm for 10, 17, and 18, respectively. Intermolecular covalent bonding interactions were investigated with single crystals of tetraphenyl derivatives 19 and 20. X-ray analysis revealed that 19 and 20 afforded 1D chains in a slipped stacking arrangement with the superimposed phenalenyl moieties overlapping, which were almost identical to that of 13 (Figure 30).[58,59] The very short π–π contacts (3.16 Å for 19, 3.15 Å for 20) indicate the adequate covalent bonding interaction between molecules. The solid 19 showed a red-shift of the lowest-energy band in the UV–vis–NIR spectrum with respect to the solution band of 17, like 13. Compounds 20 and 18 also gave similar results. However, the solid-state bands of 19 and 20 were located at a

Chem. Rec. 2015, 15, 218–232

Fig. 31. The lowest-energy absorption bands of solid 13 (black line), 19 (red line), and 20 (blue line).

higher-energy region (centered at 1280 nm for 19 and 1025 nm for 20) relative to that of 13 (1440 nm) (Figure 31). As mentioned above, the 1D chain of 13 is theoretically predicted to possess a stronger covalent bonding interaction between the molecules than within the molecule. Extrapolation of this theoretical result leads to the prediction that the 1D chains of 19 and 20 also have a similar state but with more enhanced intermolecular covalent bonding interactions, because 19 and 20 have a larger spin density on the phenalenyl rings than 13 on the basis of the theoretical calculations. The higher-energy bands of 19 and 20 relative to that of 13 can be explained in terms of how the intra- and intermolecular covalent bonding interactions are more unbalanced in strength in the 1D chains of 19 and 20. Recalling the mimicking of an infinite polyene, the more enhanced “bond alternation” causes a greater separation in energy between the HOMO and LUMO of the 1D chain. The stronger interaction between molecules than within a molecule could be directly observed by polarized reflection measurements on a crystal of 20.[58] We carefully investigated the intensities of the lowest-energy band (1025 nm) by rotating polarized light in steps of 30° on the (010) surface. The peak had a maximum intensity at the polarization direction parallel to the c axis, and was gradually depressed by rotating the



Personal Account

THE CHEMICAL RECORD

Fig. 32. Schematic drawing of the 1D stack of 20. Phenyl groups are omitted for clarity. Green half-arrows represent magnetic spins. Blue and red dotted lines represent the intra- and intermolecular covalent bonding interactions, respectively.

polarization direction until finally disappearing when the polarization direction was perpendicular to the c axis. If a covalent bonding interaction operates only within a molecule or between molecules, then the direction of the transition moment should be parallel (along the blue dotted line in Figure 32) or perpendicular (along the red dotted line in Figure 32) to the molecule, respectively. In the case of the well-balanced 1D stack, the transition moment for the lowestenergy band is directed along the line connecting between the centers of gravity of the molecule. The transition moment in the 1D stack of 20 tilts to the perpendicular direction (the red dotted line direction) with respect to the line connecting between the centers of gravity of the molecule. This finding experimentally demonstrates that a covalent bonding interaction in the 1D stack of 20 is substantially strong between molecules compared to within a molecule.

determined experimentally from two-photon absorption (TPA), UV–vis–NIR, and ESR (or phosphorescence, SQUID) measurements, respectively. The amount of singlet biradical character experimentally estimated for 10 was 34%, which is in good agreement with the value calculated by broken-symmetry UB3LYP calculations (37%). It is notable that TIPS-pentacene was found to have non-negligible singlet biradical character of 15%. The high reactivity of pentacene might be related partially to the biradical character in the ground state.

5. Applications of Singlet Biradicals 5.1. Field-Effect Transistor (FET) Through the long-range covalent bonding interactions, the solid 13 is invested with an electroconductive property.[54,61] The electroconductivity of a compressed pellet of 13 at room temperature was 1.0 × 10−5 S cm−1, with an activation energy of 0.3 eV at 200–300 K. It is noted that the conductivity is obtained in the single-component state, not a charge-transfer complex or salt, and is substantially large among structurally well-defined hydrocarbon molecules. An extended HMO calculation gave a very large dispersion in the valence and conduction bands along the stacking direction, fully supporting the conductive behavior. Thin-film properties and ambipolar transport have also been investigated. The organic field-effect transistors (OFETs) based on 13 exhibited ambipolar transport with balanced hole and electron mobilities in the order of 10−3 cm2/V s (Figure 33).[62]

4.6. Experimental Estimation of the Amount of Singlet Biradical Character (y) The amount of singlet biradical character (y) of a given molecule can be determined by quantum chemical calculations, although y strongly depends on calculation level and methods. Experimental evaluation of y has long been demanded from experimental chemists. Nakano, Kamada, and I demonstrated that the y value can be deduced from the equation y = 1 − 4|t|/(U2 + 16t2)1/2, where U represents the difference between on-site and inter-site Coulomb integrals and t is a transfer integral, using the localized natural orbital basis.[60] This formula is alternatively expressed as:

⎛ E S1u ,S1g − ET 1u ,S1g ⎞ y = 1− 1− ⎜ ⎟⎠ E S 2 g ,S1g ⎝

2

Weak coupling of two unpaired electrons allows the electronic distribution along the π-conjugated system to distort easily. Nakano theoretically found that molecules with an intermediate singlet biradical character feature an enhanced third-order non-linear optical response and, consequently, a large twophoton absorption activity is expected.[63–66] Kamada indeed measured the TPA activity of some singlet biradicals including 10, 13 and 17, and found that they have much stronger TPA activity than closed-shell π-conjugated hydrocarbons with similar molecular size.[67] The TPA crosssectional values determined are listed in Table 1.

(1)

where ES2g,S1g, ES1u,S1g, and ET1u,S1g correspond to the excitation energies of the higher singlet state of g symmetry (two-photon allowed excited state), of the lower singlet state with u symmetry (one-photon allowed excited state), and of the triplet state with u symmetry, respectively. These parameters can be

Chem. Rec. 2015, 15, 218–232

5.2. Non-Linear Optical Properties

6. Conclusion and Outlook Interest in the chemistry of phenalenyl is still growing. Although application to functional materials is an important mission, it is quite certain that many fundamental questions remain to be answered. For instance, the parent phenalenyl



Personal Account

THE CHEMICAL RECORD

Fig. 33. Output characteristics of the ambipolar OFET based on 13 for negative (left) and positive gate (right).

Table 1. Two-photon absorption properties of hydrocarbons. Compound 10 13 17 TIPS-pentacene Diphenyloctatetraene Bis(o-methylstyryl)benzene

σ(2)[a] [GM]

330 424 890[c] 27 61 66

λ(2)[b] [nm]

1300 1425 1500 875 608 590

[a] Peak two-photon absorption cross-section. [b]Wavelength of the two-photon peak. [c]Maximum value observed in the one-photon off-resonance region.

radical is known to be in equilibrium with a dimer, but its structure has not yet been identified. One of the recent topics is to understand the nature of chemical bonds in molecular aggregates of phenalenyl radicals. Treatment of weak electron–electron coupling requires highlevel quantum chemical calculations, which interests theoreticians. From the experimental side, it is very challenging to construct a phenalenyl trimer or higher aggregates, which will be associated with the fundamental character, especially the way of covalent bonding interaction, of neutral radicals and furthermore with the quest for exotic genuinely organic molecular functionalities.

Acknowledgements I deeply thank Professor Ichiro Murata and Professor Kazuhiro Nakasuji for continuous guidance and helpful suggestions. I also thank Professor Robert C. Haddon, Professor Kagetoshi Yamamoto, and Dr. Yasukazu Hirao for continuing interest and guidance in this work. I am grateful to all of my graduate

Chem. Rec. 2015, 15, 218–232

students for their restless efforts. I also thank my collaborators, Professor Kenji Kamada of the National Institute of Advanced Industrial Science and Technology (AIST) for TPA measurements; Professor Takeji Takui, Professor Kazunobu Sato, and Professor Daisuke Shiomi of Osaka City University for ESR and SQUID measurements; Professor Kyuya Yakushi and Dr. Mikio Uruichi of the Institute for Molecular Science (IMS) for polarized reflection measurements; Dr. Masayuki Chikamatsu and Dr. Yuji Yoshida of AIST for OFET measurements; and, in particular, Professor Masayoshi Nakano of Osaka University for theoretical studies on singlet biradicals. Discussions with him are always inspirational and fruitful.

REFERENCES [1] [2]

[3] [4]

[5] [6] [7] [8]

D. H. Reid, Q. Rev., Chem. Soc. 1965, 19, 274–302. Y. Morita, S. Nishida, in Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds (Ed.: R. Hicks), Wiley-Blackwell, New York, 2010, Chapter 3. C. A. Coulson, G. S. Rushbrooke, Proc. Cambridge Philos. Soc. 1940, 36, 193–200. a) C. A. Coulson, H. C. Longuet-Higgins, Proc. R. Soc. London, Ser. A 1947, 191, 39–60; b) C. A. Coulson, H. C. Longuet-Higgins, Proc. R. Soc. London, Ser. A 1947, 192, 16–32; c) C. A. Coulson, H. C. Longuet-Higgins, Proc. R. Soc. London, Ser. A 1948, 193, 447–464; d) C. A. Coulson, H. C. Longuet-Higgins, Proc. R. Soc. London, Ser. A 1948, 195, 188–197. H. C. Longuet-Higgins, J. Chem. Phys. 1950, 18, 265–274. A. Sarkar, F. S. Tham, R. C. Haddon, J. Mater. Chem. 2011, 21, 1574–1581, and references therein. H. M. McConnell, J. Chem. Phys. 1963, 39, 1910. Y. Takano, T. Taniguchi, H. Isobe, T. Kubo, Y. Morita, K. Yamamoto, K. Nakasuji, T. Takui, K. Yamaguchi, J. Am. Chem. Soc. 2002, 124, 11122–11130.



Personal Account

[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

[20] [21] [22] [23]

[24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]

[36]

THE CHEMICAL RECORD

D. Small, V. Zaitsev, Y. Jung, S. V. Rosokha, M. Head-Gordon, J. K. Kochi, J. Am. Chem. Soc. 2004, 126, 13850–13858. B. Kolb, M. Kertesz, T. Thonhauser, J. Phys. Chem. A 2013, 117, 3642–3649. T. Nozoe, Yakugaku (Sci. Drugs) 1949, 3, 174–198. T. Nozoe, T. Mukai, I. Murata, J. Am. Chem. Soc. 1954, 76, 3352–3353. I. Murata, T. Nakazawa, M. Okazaki, Tetrahedron Lett. 1969, 10, 1921–1924. I. Murata, T. Nakazawa, S. Tada, Tetrahedron Lett. 1971, 12, 4799–4802. K. Nakasuji, M. Yamaguchi, I. Murata, H. Nakanishi, J. Am. Chem. Soc. 1986, 108, 325–327. S. Akiyama, K. Nakasuji, K. Akashi, M. Nakagawa, Tetrahedron Lett. 1968, 9, 1121–1126. K. Nakasuji, S. Akiyama, M. Nakagawa, Bull. Chem. Soc. Jpn. 1972, 45, 883–891. K. Nakasuji, T. Kubo, Bull. Chem. Soc. Jpn. 2004, 77, 1791– 1801. K. Goto, T. Kubo, K. Yamamoto, K. Nakasuji, K. Sato, D. Shiomi, T. Takui, M. Kubota, T. Kobayashi, K. Yakusi, J. Am. Chem. Soc. 1999, 121, 1619–1620. F. Mota, J. S. Miller, J. J. Novoa, J. Am. Chem. Soc. 2009, 131, 7699–7707. S. Suzuki, Y. Morita, K. Fukui, K. Sato, D. Shiomi, T. Takui, K. Nakasuji, J. Am. Chem. Soc. 2006, 128, 2530–2531. R. C. Haddon, Nature 1975, 256, 394–396. P. A. Koutentis, Y. Chen, Y. Cao, T. P. Best, M. E. Itkis, L. Beer, R. T. Oakley, A. W. Cordes, C. P. Brock, R. C. Haddon, J. Am. Chem. Soc. 2001, 123, 3864–3871. P. Bag, F. S. Tham, B. Donnadieu, R. C. Haddon, Org. Lett. 2013, 15, 1198–1201. S. K. Pal, M. E. Itkis, F. S. Tham, R. W. Reed, R. T. Oakley, R. C. Haddon, Science 2005, 309, 281–284. F. Hund, Linienspektren und Periodisches System der Elemente, Springer-Verlag, Berlin/Heidelberg, 1927, p. 124. H. M. McConnell, Proc. Robert A. Welch Found. Conf. Chem. Res. 1967, 11, 144. H. A. Jahn, E. Teller, Proc. R. Soc. London, Ser. A 1937, 161, 220–235. T. Kubo, K. Yamamoto, K. Nakasuji, T. Takui, I. Murata, Angew. Chem. Int. Ed. Engl. 1996, 35, 439–441. T. Kubo, K. Yamamoto, K. Nakasuji, T. Takui, I. Murata, Bull. Chem. Soc. Jpn. 2001, 74, 1999–2009. F. Gerson, Helv. Chim. Acta 1966, 49, 1463–1467. W. Broser, H. Kurreck, S. Oestreich-Janzen, G. Schlömp, H.-J. Fey, B. Kirste, Tetrahedron 1979, 35, 1159–1166. R. C. Haddon, F. Wudl, M. L. Kaplan, J. H. Marshall, R. E. Cais, F. B. Bramwell, J. Am. Chem. Soc. 1978, 100, 7629–7633. U. Grieser, K. Hafner, Chem. Ber. 1994, 127, 2307–2314. Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, K. Nakasuji, Nat. Mater. 2007, 7, 48–51. K. Uchida, Y. Hirao, H. Kurata, T. Kubo, S. Hatano, K. Inoue, Chem. Asian J. 2014, 9, 1823–1829.

Chem. Rec. 2015, 15, 218–232

[37]

[38] [39] [40] [41]

[42] [43] [44]

[45] [46]

[47] [48] [49] [50] [51] [52] [53] [54]

[55] [56] [57]

[58] [59]

[60]

[61]

T. Kubo, Y. Katada, A. Shimizu, Y. Hirao, K. Sato, T. Takui, M. Uruichi, K. Yakushi, R. C. Haddon, J. Am. Chem. Soc. 2011, 133, 14240–14243. L. Noodleman, J. Chem. Phys. 1981, 74, 5737–5743. D. Döhnert, J. Koutecký, J. Am. Chem. Soc. 1980, 102, 1789– 1796. K. Yamaguchi, Chem. Phys. Lett. 1975, 33, 330–335. K. Yamaguchi, in Self-Consistent Field: Theory and Applications (Eds.: R. Carbo, M. Klobukowski), Elsevier, Amsterdam, 1990, pp. 727–823. S. Yamanaka, M. Okumura, M. Nakano, K. Yamaguchi, J. Mol. Struct.: THEOCHEM 1994, 310, 205–218. I. Murata, S. Sasaki, K.-U. Klabunde, J. Toyoda, K. Nakasuji, Angew. Chem. Int. Ed. Engl. 1991, 30, 172–173. K. Ohashi, T. Kubo, T. Masui, K. Yamamoto, K. Nakasuji, T. Takui, Y. Kai, I. Murata, J. Am. Chem. Soc. 1998, 120, 2018– 2027. J. E. Douglas, B. S. Rabinovitch, F. S. Looney, J. Chem. Phys. 1955, 23, 315–323. T. Kubo, M. Sakamoto, M. Akabane, Y. Fujiwara, K. Yamamoto, M. Akita, K. Inoue, T. Takui, K. Nakasuji, Angew. Chem. Int. Ed. 2004, 43, 6474–6479. M. S. Platz, in Diradicals (Ed.: W. T. Borden), Wiley, New York, 1982, pp. 195–258. J. Kolc, J. Michl, J. Am. Chem. Soc. 1973, 95, 7391–7401. C. R. Flynn, J. Michl, J. Am. Chem. Soc. 1974, 96, 3280– 3288. J. Kolc, J. W. Downing, A. P. Manzara, J. Michl, J. Am. Chem. Soc. 1976, 98, 930–937. L. Salem, C. Ronland, Angew. Chem. Int. Ed. Engl. 1972, 11, 92–111. P. C. Hiberty, P. Karafiloglou, Theor. Chim. Acta 1982, 61, 171–177. V. Bonacˇic´-Koutecký, J. Koutecký, J. Michl, Angew. Chem. Int. Ed. Engl. 1987, 26, 170–189. T. Kubo, A. Shimizu, M. Sakamoto, M. Uruichi, K. Yakushi, M. Nakano, D. Shiomi, K. Sato, T. Takui, Y. Morita, K. Nakasuji, Angew. Chem. Int. Ed. 2005, 44, 6564–6568. W. H. Watson, R. P. Kashyap, B. F. Plummer, W. G. Reese, Acta Crystallogr. 1991, C47, 1848–1851. J. Huang, M. Kertesz, J. Am. Chem. Soc. 2007, 129, 1634– 1643. T. Kubo, A. Shimizu, M. Uruichi, K. Yakushi, M. Nakano, D. Shiomi, K. Sato, T. Takui, Y. Morita, K. Nakasuji, Org. Lett. 2006, 9, 81–84. A. Shimizu, Y. Hirao, K. Matsumoto, H. Kurata, T. Kubo, M. Uruichi, K. Yakushi, Chem. Commun. 2012, 48, 5629–5631. A. Shimizu, T. Kubo, M. Uruichi, K. Yakushi, M. Nakano, D. Shiomi, K. Sato, T. Takui, Y. Hirao, K. Matsumoto, H. Kurata, Y. Morita, K. Nakasuji, J. Am. Chem. Soc. 2010, 132, 14421– 14428. K. Kamada, K. Ohta, A. Shimizu, T. Kubo, R. Kishi, H. Takahashi, E. Botek, B. Champagne, M. Nakano, J. Phys. Chem. Lett. 2010, 1, 937–940. H. Koike, T. Kubo, K. Uchida, M. Chikamatsu, R. Azumi, K. Mase, K. Kanai, Appl. Phys. Lett. 2013, 102, 134103.



Personal Account

[62]

[63]

[64]

[65]

THE CHEMICAL RECORD

M. Chikamatsu, T. Mikami, J. Chisaka, Y. Yoshida, R. Azumi, K. Yase, A. Shimizu, T. Kubo, Y. Morita, K. Nakasuji, Appl. Phys. Lett. 2007, 91, 043506. M. Nakano, R. Kishi, T. Nitta, T. Kubo, K. Nakasuji, K. Kamada, K. Ohta, B. Champagne, E. Botek, K. Yamaguchi, J. Phys. Chem. A 2005, 109, 885–891. M. Nakano, T. Kubo, K. Kamada, K. Ohta, R. Kishi, S. Ohta, N. Nakagawa, H. Takahashi, S. Furukawa, Y. Morita, K. Nakasuji, Chem. Phys. Lett. 2006, 418, 142–147. M. Nakano, R. Kishi, N. Nakagawa, S. Ohta, H. Takahashi, S. Furukawa, K. Kamada, K. Ohta, B. Champagne, E. Botek, S.

Chem. Rec. 2015, 15, 218–232

[66]

[67]

Yamada, K. Yamaguchi, J. Phys. Chem. A 2006, 110, 4238– 4243. S. Ohta, M. Nakano, T. Kubo, K. Kamada, K. Ohta, R. Kishi, N. Nakagawa, B. Champagne, E. Botek, S.-y. Umezaki, A. Takeba, H. Takahashi, S. Furukawa, Y. Morita, K. Nakasuji, K.Yamaguchi, Chem. Phys. Lett. 2006, 420, 432–437. K. Kamada, K. Ohta, T. Kubo, A. Shimizu, Y. Morita, K. Nakasuji, R. Kishi, S. Ohta, S.-I. Furukawa, H. Takahashi, M. Nakano, Angew. Chem. Int. Ed. 2007, 46, 3544–3546.



Distributions of polycyclic aromatic hydrocarbons and alkylated polycyclic aromatic hydrocarbons in Osaka Bay, Japan.

Polycyclic aromatic hydrocarbons in human bronchial carcinoma.

Metabolism of nitro-polycyclic aromatic hydrocarbons.

Extended O-Doped Polycyclic Aromatic Hydrocarbons.

Polycyclic aromatic hydrocarbons (PAHs) in yogurt samples.

Detoxification of polycyclic aromatic hydrocarbons by fungi.

Polycyclic aromatic hydrocarbons and childhood asthma.

Aza-polycyclic aromatic hydrocarbons from Saruma henryi.

Multiphoton ionization mass spectrometry of nitrated polycyclic aromatic hydrocarbons.

Effect of rhamnolipid biosurfactant on solubilization of polycyclic aromatic hydrocarbons.

Biodegradation of polycyclic aromatic hydrocarbons by Novosphingobium pentaromativorans US6-1.

Activity of Arylsulphatase in Soil Contaminated with Polycyclic Aromatic Hydrocarbons.

Polycyclic Aromatic Hydrocarbons in drinking water of Tehran, Iran.

Polycyclic aromatic hydrocarbons in olive oils on the Italian market.

Carcinogenic activity of polycyclic aromatic hydrocarbons bounded on particle fraction.

Exposure of firefighters to particulates and polycyclic aromatic hydrocarbons.

Effects of three polycyclic aromatic hydrocarbons on sediment bacterial community.

Accumulation of polycyclic aromatic hydrocarbons in cultivated microalgae.

Vibrational spectroscopic study of vinyl substituted polycyclic aromatic hydrocarbons.

Biomarkers for polycyclic aromatic hydrocarbons and serum liver enzymes.

Polycyclic aromatic hydrocarbons: levels and phase distributions in preschool microenvironment.

Polycyclic aromatic hydrocarbons, tobacco smoke, and epigenetic remodeling in asthma.

Determination of polycyclic aromatic hydrocarbons in white petroleum products.

Degeneration of mouse oocytes in response to polycyclic aromatic hydrocarbons.